This invention relates generally to an antenna apparatus, system and method for receiving and transmitting cellular telephone signals. More particularly, the invention relates to a dipole antenna coupled to a transmission line that is printed on a vehicle window.
A number of apparatus and methods exist for an antenna that utilizes the surface of a glass. For example, one type of antenna has been used exclusively for reception in the VHF band, having a low gain and an unfavorably high voltage standing wave ratio (VSWR). For practical reasons, pole or rod antennas have been used for portable communications services such as cellular telephones and for receiving global positioning satellite (GPS) signals.
Rod and pole antenna typically extend outward from the automobile, and generally create noise at high speed, interfere with washing of the vehicle, can be snagged on low branches, and adversely affect the overall aesthetics of the vehicle.
Dipole antennas typically appear as a metal rectangle on the end of a short mounting beam, and is the basic antenna for fixed point communications. Dipole antennas are omni-directional when vertically polarized and have relatively low gain. It is not common to use a dipole antenna in a horizontally polarized system because other antennas having higher gain and lower cost are readily available.
As depicted in FIG. 1, shielded dipole antennas 10 are also known, for example, U.S. Pat. No. 4,746,925. The coaxial cable 12 must run across a window glass 14, which is aesthetically unappealing and obscures driver or passenger visibility. Moving the antenna 10 closer to pillars or trim area 16 degrade performance as the dipole radiation pattern is severely distorted by the proximity of the surrounding metal, as well as significantly radiating into the vehicle.
Consequently, there is a need for a dipole antenna that provides improved antenna performance and as well as improved aesthetic qualities.
Accordingly, it is a primary object of the present invention to provide an antenna system for the reception of cellular telephone signals and transmission of the cellular telephone signals to a cellular receiver, as well as the transmission of cellular signals from a cellular transmitter to external cellular receivers over a transmission line having an improved omni-directional antenna pattern when mounted on a vehicle window.
Another object of the invention is to provide a dipole antenna mounted on the surface of a vehicle window that is in a clear path RF environment.
Another object of the invention utilizes two sets of dipole antennas with a modified feed length, each antenna positioned on opposite sides of the vehicle providing enhancement of the radiation pattern.
The present invention is directed to an automotive on glass antenna having parallel tuned feeders. Two sets of antenna elements are printed on a vehicle window and are tuned to an upper part of the desired frequency band and to a lower part of the desired frequency band. The antenna elements can be printed on the glass using techniques known in the art for printing rear defogger elements and AM/FM radio antennas onto glass. For example, in a cellular telephone application having a bandwidth of approximately 70 MHz, a VSWR of less than 2:1 can be maintained. Each tuned dipole antenna employs three elements to broad band the dipole antenna. A parallel tuned feeder for each antenna is a multiple electrical half wavelength to transfer the approximately 50 Ohm impedance of the dipole. Parallel tuned feeders transform the impedance of the coaxial cable to match the impedance of the antenna. The parallel tuned feeder allows for the placement of the printed modified dipole antenna in a clear path RF environment, resulting in a well-defined omni-directional antenna pattern.
The printed antenna elements are connected to one end of a coaxial cable, which forms a coaxial transmission line. This coaxial transmission line has an impedance of approximately 75 Ohms and odd multiple electrical quarter wavelengths. One hundred-ohm transmission line combines in parallel to 50 ohms, feeding into a 50-ohm transmission line matching the impedance of the transmitter. This results in the power supplied at the feed point to be split and each antenna receives one-half of the input power.
A relatively symmetrical radiation pattern is achieved by placing one of these dipoles on each side window of a vehicle having stationary window glass, resulting in space diversity. Additionally, by splitting the power equally between the antennas, the field strength is also divided, and the amount of RF exposure to the interior of the vehicle is reduced.
One advantage of using two dipoles with space diversity is an improved radiation pattern versus a single dipole pattern.
Use of window mount dipole antenna of this invention virtually eliminate rain leakage, are less costly that roof installed antennae, improves vehicle appearance, and can be utilized on all vehicles having a stationary or partially stationary window. Vehicle appearance is also improved by concealing the coaxial transmission line going to the transmitter, for example, beneath the roof liner.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.